Sodium secondary batteries are attracting considerably renewed interest as new battery systems owing to the high and uniform abundance and cost advantages of Na. However, their performance is still far from optimal as compared to the well-developed Li-ion technology. Herein, Na secondary batteries with unprecedented rate capability and a long life has been achieved by using a highly concentrated bis(fluorosulfonyl)amide anion (FSA(-))-based ionic liquid electrolyte (3.3 mol dm(-3) Na[FSA]) and a Na2FeP2O7 positive electrode, in a targeted operating temperature range from room to intermediate. Nearly full discharge capacity is obtained at 4000 mA g(-1), and 79% of the capacity is retained at a discharge rate as high as 20000 mA g(-1) at 363 K. Stable cycling (>300 cycles) with satisfactory coulombic efficiency (>99.5%) is found at an intermediate rate (100 mA g(-1)) over 298-363 K. A high-rate cycling test (1000 mA g(-1)) at 363 K reveals that the cell could retain 93% of its initial capacity after 1500 cycles. (C) 2016 Elsevier B.V. All rights reserved.